Contemporary navigation systems commonly employ satellite navigation data derived from radio navigation receivers to determine the location of a navigation platform. The satellite data is often augmented by data from additional sensors in order to improve navigation system performance in situations where satellite data may be available only intermittently or may be degraded by intentional or unintentional interference. For example, data from inertial sensors (accelerometers and gyros) may be used to allow full navigation solutions (position, velocity and attitude) to be maintained in the presence of satellite data dropouts over extended periods of time. Radio navigation data is generated by receivers specifically designed to receive and process radio signals transmitted from a plurality of space-based or ground-based terminals. The navigation sensors are generally, but need not be, collocated on the same navigation platform. Depending on the application, the navigation system objective may be to determine position only, position and velocity, or a larger set of parameters which may include navigation platformn attitude. If required, the satellite navigation data can be used to perform error calibration of the other sensors, thereby reducing navigation system errors when the satellite data are temporarily unavailable or corrupted by interference.
Radio navigation data are processed in order to determine the line-of-sight distance of the navigation base from each transmitter in view of the receiver. This is accomplished by measuring the time of arrival of each signal, and comparing it to the known time of transmission, using a common time reference.
Contemporary radio navigation receivers used for this purpose generally include an antenna for receiving the radio signals, a front-end for amplifying, down-converting and bandpass-filtering the received signals, an analog-to-digital converter, and a signal processor. It is common practice in such receivers to generate in-phase (I) and quadrature (Q) baseband signals, which are then processed to remove noise and interference.
Noise and interference are often reduced significantly by means of spread spectrum techniques in which the transmitted signal is modulated by a known pseudo-random code. The receiver correlates the received signal with a locally-generated replica of the code and performs code tracking by varying the estimated time delay to maintain the correlation at or near its peak value. In this manner, a significant improvement in signal-to-noise ratio is obtained. If the input signal-to-noise ratio is sufficiently high, then it is possible to perform carrier phase tracking. Successful carrier tracking yields navigation accuracies which are greater than those achievable using code tracking only.
The NAVSTAR Global Positioning System (GPS), developed by the United States Government, is an example of a contemporary radio navigation system. A constellation of up to 24 satellites, positioned in precisely-known orbits, transmit pseudo-random ranging signals used by specially-designed receivers to calculate line-of-sight distance from the user receiver to any satellite in view of the receiver. Based on this information, a navigation solution can be obtained.
In modern GPS-based navigation systems, interference can have adverse effects on GPS receiver code and carrier tracking, resulting in degraded navigation system performance. Interference can be intentional or unintentional. Examples of unintentional interference include: (1) out-of-band signals from nearby transmitters with inadequate radio-frequency (rf) filtering, (2) harmonic or intermodulation products of various ground and airborne transmitters, (3) active or passive intermodulation products of signals or local oscillators on the same platform as the GPS receiver or on nearby platforms, (4) pulsed interference from radar signals in nearby frequency bands, (5) accidental interference from unlicensed transmitters. The results of interference are a reduction in signal-to-noise ratio (SNR) at the receiver input. The transmitted GPS signal may be further attenuated by trees, buildings, etc., resulting in a further reduction of SNR. Sources of intentional interference include narrowband and wideband jammers specifically designed to reduce SNR at the receiver input.
Current GPS satellites transmit on two frequencies, L1=1575.42 MHZ and L2=1227.6 MHZ. The satellites transmit their signals using spread spectrum techniques and employ two different spreading functions: (1) a 1.023 MHZ coarse/acquisition (C/A) code on L1 only and (2) a 10.23 MHZ precision P(Y) code on both L1 and L2. The minimum signal power for received GPS signals is specified as follows; for L1, C/A=xe2x88x92160 dBW (decibels with respect to one Watt), P=xe2x88x92163 dBW; for L2, P=xe2x88x92166 dBW. A typical value of equivalent received thermal noise power is xe2x88x92131 dBW. Thus, recovery of the GPS signal, even with no interference, cannot be accomplished without special design techniques such as spread spectrum.
As described above, the GPS signal is broadcast using standard spread spectrum techniques in which the narrow bandwidth signal is spread to a much larger bandwidth by using a pseudo-random code. By correlating the received signal with a known replica of the pseudo-random code and then bandpass filtering the result over the narrow signal bandwidth, the effects of interference are significantly reduced. The gain in SNR due to spread spectrum processing is on the order of 53 dB for P(Y)-code and 43 dB for C/A code in a 50 Hz bandwidth.
Outputs from other navigation sources, (for example inertial navigation system (INS) outputs, radars, altimeters, etc. . . . ) are often combined with GPS navigation system outputs to provide navigation solutions which are improved over those resulting from systems which employ either one used independently. These outputs assist in maintaining an accurate navigation solution over limited time intervals during periods of GPS dropout. The INS is also used to aid the GPS receiver in the presence of large platform accelerations, enabling narrow tracking filter bandwidths.
Current GPS-based navigation system architectures can be categorized generally as xe2x80x9cloosely coupledxe2x80x9d or xe2x80x9ctightly coupledxe2x80x9d. A loosely coupled system, for example, may combine the navigation solution generated by a GPS receiver (position, velocity, time) with the navigation solution provided by an INS navigation system (position, velocity, attitude) using a weighting scheme generally based on a Kalman filter. A minimum of four satellites are required to obtain the GPS navigation solution. A tightly-coupled system computes pseudorange and, deltarange (integral of Doppler velocity) measurements obtained by a GPS receiver, and for example, combines them with the INS navigation solution. A tightly-coupled system offers the advantage of obtaining a navigation solution with less than four satellites in view.
Vulnerability of current GPS receivers to interference has resulted in various designs for interference suppression which generally involve the use of patch antennas and/or specialized signal processors placed in front of the receiver input. Another approach uses aiding signals from inertial and/or other sensors to allow carrier/code tracking at narrow bandwidths in highly dynamic regimes such as aircraft. These methods may be regarded as ad hoc, since they only attempt to modify current systems at easily accessible points within the system. As a result, current systems are far from optimal in terms of interference rejection and navigation accuracy. There is a need for a fully integrated design which achieves near-optimal performance.
The present invention is directed to an apparatus and method for obtaining reliable and accurate GPS-based navigation solutions (optimal estimates of current position, time and/or other parameters such as velocity and attitude, as appropriate to the problem at hand) in high interference and dynamic environments, at a performance level which has heretofore been unobtainable.
The philosophy of the approach taken in the present invention differs from that of previous techniques; as a result, the navigation system architecture and processes employed yield significant improvements in navigation system performance, both in code tracking and reacquisition, and in carrier tracking and reacquisition. The improvements are particularly significant at low signal/noise ratios, where conventional approaches are especially susceptible to loss of code lock or carrier lock.
The present invention approaches the problem as a navigation problem, where the overall goal is to calculate an optimal, or xe2x80x9cbestxe2x80x9d, estimate of the components of a specified navigation state vector, given an initial estimate of the state vector and the past history of measurements. The measurements to be used are referred to herein as xe2x80x9crawxe2x80x9d measurements, consisting of a least in-phase (I) and quadrature (Q) data obtained from one or more correlators operating on received GPS signals. Additional navigation sensors may be used, depending on the application at hand, to enhance navigation system performance. These sensors include, but are not limited to, inertial sensors (gyros and accelerometers), altimeters, radars and electro-optical devices. By using the xe2x80x9crawxe2x80x9d measurements from all of the sensors, potential loss of information due to preprocessing operations is prevented.
In a preferred embodiment, the xe2x80x9cbestxe2x80x9d estimate is the minimum variance estimate, which is the conditional mean of the state vector, conditioned on the past measurement history.
The state vector estimate is updated based on the use of nonlinear stochastic differential equations for propagating the first two conditional moments of the state vector, given the past measurement history. Determination of the first conditional moment, otherwise referred to as the conditional mean, requires knowledge of the second conditional moment, which is represented by the estimation error covariance matrix. The conditional mean and error covariance matrix estimates are calculated recursively in real time in the navigation filter described herein.
The technique of the present invention is not dependent in any manner on a priori notions of tracking filters, phase lock loops, delay lock loops, and the like, which are commonly used. Instead, the navigation system architecture and processes flow naturally from the problem description and the characterization of the measurements from GPS and other sensors. This leads to an architecture which is amenable to single processor integration, eliminating the modular (and sub-optimal) architectures of current designs.
The resulting navigation filter processes are significant departures from traditional Kalman and extended Kalman filter algorithms generally used for GPS-based navigation. The differences arise principally from the fact that the measurement functions are highly nonlinear. Whereas previous designs rely on linearization techniques, the present invention takes full account of measurement nonlinearities. As a result, the error covariance matrix is driven by the measurements and the state vector gain matrix is a nonlinear function of the estimated error covariance matrix.
A bank of correlators and a processing architecture based on nonlinear estimation techniques are employed to maintain code track: through extremely high jamming environments. The number of correlators are optionally extendable, to allow for optimal code loop reacquisition following GPS signal dropout, eliminating the need for time-consuming reacquisition as employed by conventional direct-search techniques. The outputs of each correlator in the correlator bank are preferably weighted to provide a fully optimized navigation solution.
The present invention extends the range of delay error beyond the linear range through the use of multiple correlators. The number of correlators is arbitrary. The need for moding logic, which depends on estimated carrier/noise (C/N0) ratio is eliminated. Current receivers employ fixed gain filters, depending on the mode of operation. The present invention, however, allows for seamless operation at any C/N0 ratio, and therefore, mode switching logic is not required.
In a first embodiment, the present invention is directed to a system for determining a multidimensional navigation state for a navigation platform based on radio navigation data transmitted by external terminals. A radio navigation data receiver receives and amplifies the radio navigation data, which arrives embedded in noise. An extended-range bank of correlators extract the radio navigation data from the noise over an extended range of delay error. The correlators process the radio navigation data and noise to generate output functions indicative of the degree of correlation for each correlator in the extended-range bank between the radio navigation data and a known internally-generated replica signal. An estimator estimates the radio navigation data signal power and noise power from the correlator output functions. A measurement update unit updates the navigation state estimate based on the estimated radio navigation data signal and noise power estimates and the correlator outputs.
In a second embodiment, the present invention is directed to a system for determining a multidimensional navigation state for a navigation platform based on radio navigation data transmitted by external terminals and based on inertial data indicative of the inertial behavior of the platform. A radio navigation data receiver receives and amplifies the radio navigation data, which arrives embedded in noise. An extended-range bank of correlators extract the radio navigation data from the noise over an extended range of delay error. The correlators process the radio navigation data and noise to generate correlator output functions indicative of the degree of correlation for each correlator in the extended-range bank between the radio navigation data and a known internally-generated replica signal. An estimator estimates the radio navigation data signal power and noise power from the correlator output functions. An inertial sensor unit generates inertial data indicative of the inertial behavior of the platform. A measurement update unit updates the navigation state based on the estimated radio navigation data signal power and noise power, the correlator outputs, and the inertial data.
The radio navigation data preferably comprises Global Positioning System (GPS) data, and the radio navigation data receiver comprises a GPS receiver. The system preferably further includes a propagator for applying sensed inertial data to a dynamic model based on a recently updated navigation state to generate updated inertial data, which is in turn provided to the measurement update unit. The multidimensional navigation state may include, for example, information related to platform position, velocity, and attitude, receiver clock errors, inertial sensor errors, propagation delays, and satellite errors.
In a preferred embodiment, the measurement update unit updates the navigation state by calculating the conditional moments of the navigation state based on the estimated radio navigation data signal power and noise power, the correlator outputs, and the inertial data.
The conditional moments may comprise a first conditional moment comprising a conditional mean, and a second conditional moment comprising a conditional error covariance matrix. The conditional moments are preferably calculated to include measurement non-linearities.
The estimator preferably comprises a signal power estimator for determining radio navigation data signal power based on a nonlinear conditional moment of the navigation state, and a noise power estimator for estimating noise power based on a nonlinear conditional moment of the navigation state, and the signal power estimate.
In a third aspect, the present invention is directed to a system for determining a multidimensional navigation state for a navigation platform based on radio navigation data transmitted by external terminals. A radio navigation data receiver receives and amplifies the radio navigation data, which is embedded in noise. A bank of correlators extract the radio navigation data from the noise over a range of delay error. The correlators process the radio navigation data and the noise to generate correlator output functions indicative of degree of correlation for each correlator between the radio navigation data and a known internally-generated replica signal. An estimator estimates the radio navigation data signal power and noise power from the correlator output functions. A measurement update unit updates the navigation state based on the conditional moments of the navigation state calculated as a function of the estimated radio navigation data signal power and noise power, and the correlator output functions.
In a preferred embodiment, the third aspect of the present invention further comprises an inertial sensor unit for generating inertial data indicative of the inertial behavior of the platform. In this embodiment, the conditional moments of the navigation state are preferably further calculated as a function of the inertial data. The conditional moments preferably comprise a first conditional moment comprising a conditional mean, and a second conditional moment comprising a conditional error covariance matrix. The conditional moments are preferably calculated to include measurement nonlinearities. The bank of correlators preferably comprise an extended-range bank of correlators for extracting the radio navigation data from the noise over an extended range of delay error.
In a fourth aspect, the present invention is directed to an estimator for estimating signal and noise power in a system for determining a multidimensional navigation state for a navigation platform based on radio navigation data embedded in noise. The radio navigation data signal is received by a radio navigation data receiver including a bank of correlators for extracting the radio navigation data from the noise over a range of delay error. The correlators process the radio navigation data and the noise to generate correlator output functions indicative of the degree of correlation for each correlator between the radio navigation data and a known internally-generated replica signal. The estimator includes a signal power estimator for determining radio navigation data signal power based on a conditional moment of the navigation state, a noise power estimator for estimating noise power based on a conditional moment of the navigation state, and the signal power estimate.
In a preferred embodiment of the fourth aspect of the present invention, the estimator further includes a bias estimator for determining a bias estimate of each correlator in the bank. In this arrangement, the signal power estimate and noise power estimate are further based on the bias estimate. The bias estimator preferably comprises a smoothing function for updating the bias estimate of each correlator according to a smoothing time constant; and a minimum function for determining the bias estimate as the minimum value of the smoothed bias estimates.
The conditional moment preferably comprises a first conditional moment comprising a conditional mean, calculated by means of a nonlinear filter. The resulting noise power estimate preferably comprises a vector, such that an independent noise power estimate is calculated for each correlator in the correlator bank. The bank of correlators preferably comprises an extended-range bank of correlators for extracting the radio navigation data from the noise over an extended range of delay error. An inertial sensor unit is preferably included for generating inertial data indicative of the inertial behavior of the platform. The conditional moment of the navigation state is preferably further calculated as a function of the inertial data.
In a fifth aspect, the present invention is directed to a measurement update unit for updating the navigation state and error covariances of the navigation state for a navigation platform based on radio navigation data embedded in noise. The radio navigation data signal is received by a radio navigation data receiver including a bank of correlators for extracting the radio navigation data from the noise over a range of delay error. The correlators process the radio navigation data and the noise to generate correlator output functions indicative of degree of correlation for each correlator between the radio navigation data and a known internally-generated replica signal. A state update unit updates the navigation state as a function of the correlator output functions weighted by state gain functions. The state gain functions are based on conditional moments of nonlinear functions of the navigation state, and an inertial data propagation of a previous error covariance of the navigation state. An error covariance matrix update unit updates the error covariance of the navigation state based on conditional moments of nonlinear functions of the navigation state, and further based on estimates of the radio navigation data signal power and noise power.
In a preferred embodiment, the conditional moments comprise statistical expectations of nonlinear functions of the line-of-sight position error for each satellite in view.
According to the fifth aspect, the bank of correlators preferably comprises an extended-range bank of correlators for extracting the radio navigation data from the noise over an extended range of delay error. The radio navigation data signal power estimate and noise power estimate are preferably calculated by an estimator comprising a signal power estimator for determining radio navigation data signal power based on a conditional moment of the navigation state; and a noise power estimator for estimating noise power based on a conditional moment of the navigation state, and the signal power estimate.
In a sixth aspect, the present invention is directed to a measurement update unit for updating the navigation state and error covariances of the navigation state for a navigation platform based on radio navigation data carried on a radio navigation data carrier signal embedded in noise. The radio navigation data signal is received by a radio navigation data receiver including a bank of correlators for extracting the radio navigation data from the noise over a range of delay error. The correlators process the radio navigation data and the noise to generate correlator output functions indicative of degree of correlation for each correlator between the radio navigation data and a known replica signal. The bank of correlators include a prompt correlator for processing radio navigation data signals near the radio navigation data code offset. The measurement update unit comprises, a state update unit and an error covariance matrix update unit. The state update unit updates the navigation state as a function of the prompt correlator output function weighted by state gain functions. The state gain functions are based on conditional moments of a non-linear function of line-of-sight error of the radio navigation data, and an error covariance matrix. The error covariance matrix update unit updates the error covariances of the navigation state based on the state gain functions.
In a preferred embodiment of the sixth aspect of the present invention, the state gain functions are further based on estimates of the radio navigation data carrier signal power and noise power, a data bit estimate, an inertial data propagation of a previous navigation state, and an error covariance matrix. Noise power is preferably calculated based on the average noise power and at least two non-prompt correlator output functions. The state update unit and error covariance matrix update unit preferably operate linearly for a relatively large radio navigation data carrier signal power to noise power ratio and operate non-linearly for a relatively small radio navigation data carrier signal power to noise power ratio.
In a seventh aspect, the present invention is directed to a measurement update unit for updating the navigation state and error covariances of the navigation state for a navigation platform based on radio navigation data embedded in noise. The radio navigation data signal is received by a radio navigation data receiver including a bank of correlators for extracting the radio navigation data from the noise over a range of delay error. The correlators process the radio navigation data and the noise to generate correlator output functions indicative of degree of correlation for each correlator between the radio navigation data and a known replica signal. The measurement update unit comprises a state update unit, an error covariance matrix update unit, and an integrity management unit. The state update unit updates the navigation state as a function of the correlator output functions weighted by state gain functions. The state gain functions are based on conditional moments of a non-linear function of line-of-sight error of the radio navigation data, an internal data propagation of a previous navigation state, and an error covariance matrix. The error covariance matrix update unit updates the error covariances of the navigation state based on conditional moments of a non-linear function of line-of-sight error. The integrity management unit maintains the integrity of the navigation state and the error covariance.
In a preferred embodiment of the seventh aspect, the state gain functions and error covariances of the navigation state are further based on estimates of the radio navigation data signal power and noise power. The conditional moments preferably comprise non-linear functions of a line-of-sight error variance, in turn a function of the error covariance of the navigation state, and the integrity management unit preferably monitors and limits the signal and noise power values. The integrity management unit preferably further limits growth rate of the error covariances. Inertial data is preferably sensed by inertial data sensors and the growth rate of the error covariances are limited to an upper bound based on estimated accuracy of the inertial sensors. The integrity management unit may further monitor and maintain the positive-definiteness of the error covariance matrix, and may further limit estimated change of the navigation state.
The invention is amenable to a variety of commercial applications that rely on GPS navigation, including airliners, nautical vessels, personnel, and automobiles.